2007-11-27ย ยท If the part you're painting is going to be exposed to considerable sunlight, you should topcoat it. Either use a POR-15 product directly on top of the Rust Preventive paint (within 24 hours) or use the POR-15 Tie-Coat primer on top of the Rust Preventive and then top coat with your choice. I've used the POR-15 BlackCote product.